Combination power plants embodied as a single shaft power plant are described in the report “Single shaft combined cycle power generation systems”, 9th. Conference of Electric Power Supply Industry, Hong Kong 1992, and are previously disclosed accordingly. A generator is driven in this case by a gas turbine and a steam turbine. A coupling arranged between the steam turbine and the gas turbine is embodied in this case as a synchronized and self-actuating coupling. The gas turbine can accordingly be operated on its own. The cooling periods for the rotating principal components of the gas turbine, and also those of the steam turbine, must be allowed to elapse before being able to make a start on inspection or repair work after shutting down the combination power plant. The gas turbine rotor and the steam turbine rotor must be caused to rotate by means of a shaft-rotating device during this cooling period, in order to prevent deformation as a consequence of thermal stresses. As a rule, the gas turbine will already have cooled down to a temperature at which the inspection work can be carried out after a single day, whereas the steam turbine, on the other hand, will require a cooling period of up to a week.
A combination power plant embodied as a single shaft power plant is illustrated in DE 44 26 354 C2. A coupling arranged between the gas turbine and the steam turbine is embodied in this case in such a way that it is uncoupled in the rest condition and engages automatically only upon reaching a specific boundary speed. The combination power plant described in DE 44 26 354 C2 is embodied with two shaft-rotating devices acting as drive devices. One shaft-rotating device is provided for the purpose of causing the steam turbine rotor to rotate, whereas the second shaft-rotating device is provided for the purpose of causing the gas turbine rotor to rotate.
If the rotational speed of the steam turbine is less than the rotational speed of the gas turbine, it is possible, by using the two shaft-rotating devices, to cause both the steam turbine rotor and the gas turbine rotor to rotate independently of one another during the cooling process.
Combination power plants with only a single, common drive device are also previously disclosed. In this particular embodiment, the gas turbine is caused to rotate together with the steam turbine via the drive device for the steam turbine. The turning speeds are identical in this case. The coupling between the gas turbine assembly and the steam turbine assembly is engaged.
The axial gap of such a coupling is only a few millimeters. During the cooling process, the so-called turning operation, it is necessary to cause the steam turbine rotor to rotate in order to prevent thermally induced deformation by so doing. Inspection work on the rotor or on parts of the gas turbine situated in the vicinity of the rotor in the case of a steam turbine that is involved in turning operation are prohibited for safety reasons. In the event of a fault-related incident, the coupling could be actuated unexpectedly. As a result, a major potential risk is presented by the considerable mass inertia of the steam turbine rotor and the relatively high rotational speed during turning operation, that is to say by the high stored rotational energy, if the coupling is actuated unexpectedly in a fault-related incident.